Emulsions are very widely used in numerous technical fields, simply due to the fact that they allow long-lasting dispersion of at least two liquid-liquid immiscible phases generally by means of a surface agent (surfactant).
There are two major categories of emulsions: oil-in-water emulsions (emulsions obtained by the direct route) and water-in-oil emulsions (emulsions obtained by the inverse route). In the first case, the continuous phase is the water (droplets of oil are dispersed in this phase); and in the second case, the continuous phase is the oil wherein droplets of water are dispersed. The type of a simple, direct or inverse emulsion is fixed in the first order by the preferential solubility of the surfactant in one or other phase. Recourse to a water-soluble surfactant makes it possible to preferentially form a direct emulsion, whereas a liposoluble surfactant promotes an inverse emulsion.
Numerous cosmetic compositions are prepared from emulsions (typically simple emulsions) stabilized by surfactants.
For example, EP 0 685 227 [1] proposes a very complex system of sunscreen cosmetic compositions, of the type comprising an aqueous continuous phase, a protective system capable of filtering UV rays (Parsol® MCX, TiO2, benzophenone derivatives, etc.), a surfactant, organic solvents (lower polyols and alcohols) and at least one polymer or more particularly one crosslinked copolymer (alkyl acrylates, vinyl acetate).
FR 2 858 777 [2], for its part, proposes a stable oil-in-water emulsion containing at least one fatty substance (fatty acid esters, waxes, butters, natural—plant, animal, of marine origin—oils, synthetic oils or mineral oils, hydrogenated oils and mixtures thereof), at least one surfactant (fatty acid esters of polyglycerol, such as the ethoxylated compounds, ethoxylates of alcohols), at least one cosurfactant (polyols), and water.
However, the use of surfactants in products intended to be administered to humans (whether in topical form, oral form, or the like) can be a problem. This is because surfactants can damage cell membranes. Thus, in particular in the cosmetics field, efforts have been made to reduce the potentially harmful effects of surfactants, or even to eliminate recourse to surfactants.
In this perspective, recourse to cyclodextrins has been considered.
For example, Duchêne et al. (“Cyclodextrins and emulsions”, International Journal of Pharmaceutics, 266 (2003), 85-90 [3]) and Yu et al. (“Effect of camphor/cyclodextrin complexation on the stability of O/W/O multiple emulsions”, International Journal of Pharmaceuticals, 261 (2003), 1-8 [4]) have proposed the formulation of multiple emulsions using α-, β- and γ-cyclodextrin monomers as stabilizing agent. Two of the cyclodextrins (α-, β-) exhibit stabilizing properties during the preparation of a multiple (double) emulsion; the larger cavity of γ-cyclodextrin being unfavorable to stabilization owing to its size which is too great to result in an optimum interaction with alkyl chains.
Other emulsions based on cyclodextrin monomers have also been proposed by Inoue et al. (“Emulsion preparation using β-cyclodextrin and its derivatives acting as an emulsifier”, Chem. Pharm. Bull, 56 (9), (2008), 1335-1337 [5], “Formulation and characterisation of emulsions using β-cyclodextrin as an emulsifier”, Chem. Pharm. Bull, 56 (5), (2008), 668-671, [6] and “Preparation and characterisation of n-alkane/water emulsion stabilized by cyclodextrin”, Journal of oleo science, 58, (2), (2009), 85-90, [7]). The results indicate that, on the one hand, the addition of a cyclodextrin (α-, β- and γ-monomer) leads to the formation of a precipitate and, on the other hand, the adsorption of said precipitate at the oil-water interface is necessary in certain cases in order to stabilize emulsions.
Document WO 2008/003685 [8] proposes an emulsion based on optionally modified cyclodextrin monomers, a modified polysaccharide, a fatty substance and water.
In any event, an emulsion can be prepared, but the cyclodextrin monomers do not make it possible to stabilize it in a long-lasting manner (a precipitate forms).
There is therefore a need to develop methods capable of stabilizing emulsions without having recourse to surfactants.